Chromium chromate as an inert marker in copper oxidation

Chromium chromate films applied to commercial copper foil as corrosion inhibitors are observed to move into the copper during heat treatment in air such as occurs during lamination of printed circuit boards. Chromium depth distributions measured using secondary ion mass spectrometry for foils heated at various times and temperatures show that this change in position of the chromium is not a result of diffusion of the chromium compound. Rather, the major phenomenon is outdiffusion of copper driven by its oxidation at the surface. The temperatures examined spanned one region in which copper oxidation was the rate limiting factor in the overlayer growth and a second region in which growth of the copper oxide overlayer was diffusion limited. Implications in circuit board manufacturing are discussed. Copyright © 1986 John Wiley & Sons Ltd.